What takes up system storage

I have an iPhone 5s that can hold 12GB of storage. However, my phone keeps on telling me that I have 'no free space', which makes sense seeing as my phone also tells me in the settings app that I have used up 11.9GB out of my 12GB. But when adding up all of the data from all of my apps (including music and photos ect) it only came to just over 3GB. I understand that the IOS system itself would take up some storage but I doubt it would take 8GB worth (bit of a ripoff on Apples part if so), so what could be taking up all of this space?